For example, there 4 points [0,1], [1,2], [2,1], [1,0]
pick 3 of them: [0,1], [1,2], [2,1],
so we can build 3 vectors (edges): [1,1], [1,-1], [2,0]
these vector can build a Isosceles right triangle because vector [1,1] and [1,-1] has same length, and they are vertical (because inner product is 0).